In accordance with the trends of miniaturization and multi-functioning of electronic equipment, the miniaturization of connection terminals in the components is further progressing in recent years. Therefore, various circuit coupling adhesives with which connection of such terminals can easily be done are widely used in the field of electronics mounting. For example, they are used for junction of IC chips and a flexible printed circuit board (FPC), junction of IC chips and a glass substrate on which indium-tin-oxide (ITO) electrode circuits are formed, etc.
The circuit coupling adhesive, which is a film-like or paste-like adhesive, is sandwiched between objects to be coupled, and is heated and pressed so as to bond the objects to be coupled. That is, the resin in the adhesive flows due to the application of heat and pressure such that the gap between the opposing electrodes is sealed and at the same time a part of electro-conductive particles are sandwiched between the opposing electrodes so as to achieve electrical coupling. A circuit coupling adhesive is required to have an insulative property to increase a resistance (insulation resistance) between the adjacent electrodes arranged in a surface direction as well as a conductive property to decrease a resistance (connection resistance) between the opposing electrodes arranged in a thickness direction.
An epoxy-based thermosetting resin is mainly used as an insulative resin composition for forming a circuit coupling adhesive. For example, a composition in which a thermosetting resin such as epoxy resin or phenoxy resin is combined with a curing agent is widely used.
The circuit coupling adhesive is required of high reliability since it is used for coupling of precision equipment such as a liquid crystal display (LCD), etc. Therefore, the endurance in the environment is needed in addition to conductive/insulative properties, and such properties are evaluated, for example, by a high temperature and high humidity test, a heat cycle test, etc. Since the epoxy-based resin composition used for the circuit coupling adhesive contains a hydroxyl group in the molecule and hence has high absorption, there is a case in which a coupling failure occurs in a high temperature and high humidity test. Thus, the moisture resistance thereof is a problem to be solved. Also, there is a case in which the connection resistance at the coupled part increases in a heat cycle test because of stress due to the difference in the coefficient of thermal expansion between a substrate and an adhesive, since the resin composition has a higher coefficient of thermal expansion as compared with the substrate material.
Under such situations, in order to reduce the coefficient of thermal expansion and to improve the moisture resistance, methods for adding inorganic fillers into a resin composition are proposed. The reference patent document 1 discloses an adhesive for coupling circuit members. The disclosed adhesive is characterized in that 5 to 200 weight parts of inorganic filler resin compositions are contained in 100 weight parts of adhesive resin compositions.
The reference patent document 2 discloses an epoxy resin-based sheet-like adhesive composition consisting of epoxy resin, latent curing agent, inorganic fillers and polyethersulfone. In the disclosed adhesive composition, 5-900 weight parts of inorganic fillers are contained in 100 parts of the total amount of the epoxy resin, latent curing agent, and polyethersulfone.
Patent document 1: Japanese Patent Application Publication No. H 11-61088.
Patent document 2: Japanese Patent Application Publication No. 2000-204324.